Incandescent lamp with tantalum carbide filament and nitrogen gas atmosphere



g- 4, 1970 H. A. JOHANSEN ET INCANDESCENT LAMP WITH TANTALUM CARBIDE FIWENT AND NITROGEN GAS ATMOSPHERE Filed Jan. 18, 1968 FIG.3.

FIG

FIG.5.

INVENTORS Hermon A. Johcnsen 8 Richard Corth v ATTORNEY United States Patent Oi 3,523,207. Patented Aug. 4, 1970 3,523,207 INCANDESCENT LAMP WITH TANTALUM CARBIDE FILAMENT AND NITROGEN GAS ATMOSPHERE I Herman A. Johnsen, Monroeville, Pa., and Richard Corth, East Orange, N.J., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 18, 1968, Ser. No. 698,963 Int. Cl. H01k 7/02 US. Cl. 313-113 8 Claims ABSTRACT OF THE DISCLOSURE Incandescent la-mp incorporates a filament which principally comprises tantalum carbide and a gaseous filling which consists essentially of nitrogen as the only reactive component. Such a lamp is particularly adapted to be used as a projector lamp because of the very high temperature at which the filament can be operated, and the nitrogen atmosphere permits internal reflector systems to be used.

CROSS REFERENCE TO RELATED APPLICATIONS In copending application Ser. No. 698,962, filed concurrently herewith, and owned by the present assignee, is disclosed a method for rapidly carbiding a tantalum member. When forming a filamentary member in accordance with this method, the tantalum filament is supported in a firing container consisting essentially of carbon as the only container component which will react with tantalum and the filamentary member is then heated to a temperature of from about 1800 C. to 2500 C. with the heating atmosphere being carbon, nitrogen and an inert gas. This heating is continued until the member displays the gold color of substantially stoichiometric tantalum carbide. The resulting carbided filament contains a small amount of chemically combined nitrogen.

In copending application Ser. No. 535,815, filed Mar. 21, 1966, now Pat. No. 3,411,959 and owned by the present assignee, is disclosed an alternative method for carbiding a tantalum filament prior to its incorporation into a lamp, wherein carbon is the only reactive constituent in the carburizing atmosphere.

A structure for mounting a previously carbided filament within a lamp envelope is disclosed in copending application Ser. No. 535,835, filed Mar. 21, 1966, and

I owned by the present assignee.

BACKGROUND OF THE INVENTION Tantalum carbide for use as an incandescible member for electric lamps was suggested by Von Bolton in US. Pat. No. 915,657, dated Mar. 16, 1909. After this early Work, the emergence of tungsten as a filamentary member for incandescent lamps rendered other potential filament materials impractical. In recent years, however, considerable effort has been expended upon the development of tantalum carbide as a filamentary material because of its favorable spectral emission properties and its melting point (about 4200 K.) which is the highest known for any substance. It is of course well known that the radiation from a body is a function of the fourth power of the temperature and, in addition, the higher the temperature of a body, the greater the shift toward the shorter wavelengths which produces more radiations in the visible region and thus a more eflicient and brighter light source.

In order to carbide tantalum for use in an incandescent lamp, much of the recent effort has been devoted to carbiding the tantalum after the lamp is fabricated and this is normally accomplished by including in the lamp envelope a volatile hydrocarbon such as ethylene, for example, along with other additive gases such as hydrogen and/ or halogen. Representative of such a technique are US. Pat Nos. 2,596,469, dated May 13, 1952 and 3,002,439, dated Feb. 20, 1962.

Most projection lamps are now constructed to incorporate an internal reflector, formed either of metal or a dichroic material, such as described in US. Pat. No. 3,082,345, dated Mar. 19, 1963 and U8. Pat. No. 3,331,980, dated July 18, 1967. When such reflector systems are operated in the regenerative atmospheres which are normally used to improve the performance of tantalum carbide filaments, the operation of the lamp is impaired. This is principally due to one or more of the following: (1) the chemically regenerative atmosphere components chemically react with the reflector surface thereby both destroying the reflector and the regenerative atmosphere, (2) when hydrogen and/or hydrogen-producing compounds are used as a part of the regenerative atmosphere, the thermal conductivity of the fill gas is SUMMARY OF THE INVENTION It is the general object of the present invention to provide an incandescent lamp which incorporates a tantalum carbide filament and which operates with improved performance.

It is another object of the present invention to provide an incandescent lamp which incorporates a tantalum carbide filament and which can be used as a projection lamp adapted to operate at extremely high filament temperatures.

It is a further object to provide a projection lamp which incorporates a tantalum carbide filament in addition to a reflector optical system within the envelope, and which lamp can be operated at extremely high filament temperatures.

The foregoing objects of the invention, and other objects which will become apparent as the description proceeds, are achieved by providing an incandescent lamp which incorporates a filament principally comprising stoichiometric tantalum carbide along with a gaseous filling consisting essentially of nitrogen as the only reactive component of the gaseous filling. The nitrogen constitutes from 0.4 volume percent to volume percent of the total gaseous filling within the envelope, and any additional gaseous filling other than nitrogen consists of inert gas. Such a system is particularly adapted for use in conjunction with internal reflector systems, wherein the reflectors may be of various configurations and may be formed of metal or of dichroic material.

BRIEF DESCRIPTION OF THE DRAWINGS For a beter understanding of the invention, reference should be made to the accompanying drawings wherein:

FIG. 1 is an elevational view, partly in section, illustrating a projection lamp which incorporates a tantalum carbide filament and a nitrogen operating atmosphere;

FIG. 2 is a fragmentary perspective view of the mounting arrangement which is used to secure the filament to the lead-in wires and to effect electrical connection thereto;

FIG. 3 is a perspective view, shown partly in section, of aprojection lamp which incorporates a tantalum carbide filament and a relatively massive metallic reflector;

FIG. 4 is an elevational view, shown partly in section,

of a reflector lamp which incorporates a relatively massive dichroic reflector member;

FIG. 5 is an elevational view of a reflector type lamp wherein the reflector is fabricated of metal and is supported sufliciently close to the filament to be heated to incandescence during lamp operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A projection lamp which incorporates a tantalum carbide filament is shown in FIG. 1 and in FIG. 2 is shown an enlarged fragmentary view illustrating how the filament is mounted and connected. Briefly, the lamp 10 comprises a light-transmitting glass envelope 12 which terminates in a base 14 having pins 16 afiixed thereto for purposes of electrical contact. The pins are electrically connected to a supporting frame 18 within the envelope and arbors 20 are connected between members of the frame 18. The tantalum carbide filament 22 is mounted Within the envelope 12 by making a clamped connection between the contact support members 24 and fairly heavy tantalum carbide members or rods 26. Individual sections of the filament 24 are additionally supported by supplemental support wires 28.

Tantalum carbide is a relatively brittle material, particularly when in the form of a fine filamentary member and it is difiicult to make electrical connection and support for such a member. As disclosed in the aforementioned copending application Ser. No. 535,815, filed Mar. 21, 1966, electrical connection and support are readily made by wrapping the ends of the metallic filament, prior to carbiding, about a relatively heavy tantalum rod, which is then carbided along with the filament prior to incorporation into the lamp. During the carbiding, the overwrapped filament and rod are welded together. To mount the filaments, the contact-support members 24, which can be formed of molybdenum similarly to the supporting frame 18, are welded or otherwise affixed to the frame 18 and are mechanically clamped against the relatively heavy tantalum carbide rod or member 26 which has the filament 22 welded thereto.

In accordance with the present invention, the atmosphere within the projection lamp 10 consists essentially of nitrogen as the only reactive component, and the nitrogen constitutes from 0.4 volume percent up to 100 volume percent of the total gaseous filling within the envelope 12. Any additional gaseous filling within the envelope, other than nitrogen, is an inert gas such as argon, although other noble gases can be substituted therefor. The total gas pressure within the lamp envelope is not particularly critical but for best results, the pressure of the total gaseous filling within the envelope when the lamp is not energized should be at least 600 torr.

In its preferred form, the filament consists of a homogeneous tungsten carbide-tantalum carbide with the weight ratio of tungsten to tantalum being about 10/90. Small additions of other selected metals can be used to supplement or replace the tungsten in the preferred filament and, as an example, in US. Pat. No. 3,022,437, dated Feb. 20, 1962, it is disclosed that metals such as titanium, thorium, vanadium, niobium, molybdenum, tungsten or uranium can be used with the tantalum in making the filament. In US. Pat. No. 3,022,436, dated Feb. 20, 1962, it is disclosed that zirconium or hafnium can be alloyed with the tantalum prior to carbiding. Thus the filament as used principally comprises tantalum carbide and in its preferred form, the filament comprises a homogeneous tungsten carbide-tantalum carbide as specified hereinbefore.

While the use of a nitrogen or nitrogen-argon operating atmosphere is well known for tungten filament lamps, it has been generally accepted that when a tantalum carbide filament is used, it is necessary to operate the filament in a chemically regenerative atmosphere, in order to prevent the chemical degeneration of the tantalum carbide. In accordance with the present invention, however, When the lamp filament is substantially stoichiometric tantalum carbide, it is not necessary to use a recognized chemically regenerative atmosphere as such, or at least such an atmosphere which includes carbon hydro gen, halogen or a chemical or physical admixture of these elements as gasses. Apparently the nitrogen is reactive with respect to the filament to some degree, but its reaction is such as to preserve the stoichiometry of the tantalum carbide. In actual experimental results, nitrogen used within the teachings of the present invention gave improved results over other atmospheres which incorporated carbon, hydrogen or halogen, used either singly or mixed. Particularly when the atmosphere contains a hydrocarbon, there is some tendency to deposit carbon upon the cold filament portions, the lead-ins and the envelope during operation.

In accordance with the present invention, the nitrogen atmosphere permits the use of internal reflectors and such a lamp 30 is shown in FIG. 3. Essentially this lamp corresponds to that lamp shown in FIG. 2 of US. Pat. No. 3,194,626, dated July 13, 1965, except that the support arms 32 for the single coil filament 22a are bifurcated proximate their ends in order to effect mechanical support and electrical connection for the filament generally in the same manner as is shown in FIG. 2 of the present application. In this lamp embodiment, the reflector is operatively positioned within the envelope 12a to focus through the envelope the radiations which are generated by the filament 22a. The reflector 34 is concave and relatively massive and is formed of metal.

In FIG. 4 is shown an alternative lamp embodiment 36 Which generally corresponds to the lamp as shown in FIG. 1 of US. Pat. No. 3,331,980, dated July 18, 1967, except that the inwardly extending extremities of the support members 38 are bifurcated in order to clamp and electrically connect the filament generally in the same manner as is shown in FIG. 2 of the present application. In this embodiment, the reflector 40 is formed of dichroic material which will transmit infrared radiations therethrough while simultaneously reflecting visible light in order to minimize heat on the film.

In FIG. 5 is illustrated still another embodiment wherein the lamp 42 is generally similar to the lamp shown in FIG. 3 in US. Pat. 3,082,345, dated Mar. 19, 1963, except that the supports for the filament 22 are modified in the manner as shown in FIG. 2 of the present application. The reflector 44, which is fabricated of molybdenum for example, is positioned sufficiently close to the filament to be heated to incandescence by the filament during the lamp operation.

All of the foregoing lamp embodiments which incorporate an internal reflector are made practical by the use of the nitrogen gas fill in conjunction with the tantalum carbide filament. Lamps constructed in accordance with the present invention have been operated at a filament temperature of approximately 3560 K. (true temp.) for approximately 28 hours. This increased filament operating temperature results in an increase of 44% in projection screen lumens over conventional tungsten filament projection lamps.

If the filament is carbided in an atmosphere consisting of carbon and nitrogen as the only reactive elements, in accordance with the disclosure in aforementioned concurrently filed application Ser. No. 698,962, the filament when initially mounted within the envelope will include chemically combined nitrogen in amount of from 0.05 to 0.5 weight percent, taken with respect to the total weight of the filament. This initial inclusion of the nitrogen can have a beneficial effect upon the lamp performance when used in conjunction with the nitrogen operating atmosphere. It should be understood, however, that a filament which is initially carbided without the use of nitrogen, in accordance with the manner disclosed in the aforementioned copending application Ser. No. 535,815, filed Mar.

v 21, 1966, can also be used in the present lamp with excellent results.

It will be recognized that the objects of the invention have been achieved by providing an incandescent lamp which incorporates a tantalum carbide filament and which operates with improved performance characteristics. This lamp can readily be fabricated as a projection lamp which incorporates an internal reflector.

While preferred embodiments of the invention have been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

What is claimed is:

1. In combination with an incandescent lamp comprising a sealed light-transmitting envelope with electrical lead-in means sealed therethrough, the improvement which comprises:

(a) an incandescible filament principally comprising substantially stoichiometric tantalum carbide supported within said envelope and electrically connected to said lead-in means; and

(b) a gaseous filling within said envelope and consisting essentially of nitrogen as the only reactive component, said nitrogen constituting from 0.4 volume percent to 100 volume percent of the total gaseous filling Within said envelope, and any additional gaseous filling within said envelope other than nitrogen consisting of inert gas.

2. The lamp combination as specified in claim 1, wherein said filament consists of a homogeneous tungstentantalum carbide with the weight ratio of tungsten to tantalum being about 10/90.

3. The lamp combination as specified in claim 1, wherein the pressure of the total gaseous filling within said envelope, when said lamp is not energized, is at least 600 torr.

4. The lamp combination as specified in claim 1, wherein an internal reflector is operatively positioned within said envelope to focus through said envelope radiations which are generated by said incandescible filament. 5. The lamp combination as specified in claim 4,

wherein said reflector is concave and relatively massive and is formed of metal.

6. The lamp combination as specified in claim 4, wherein said reflector is concave and relatively massive and is formed of dichroic material.

7. The lamp combination as specified in claim 4, wherein said reflector is metal and is supported sufiiciently close to said filament to be heated to incandescence by said filament during lamp operation.

8. In combination with an incandescent lamp comprising a sealed light-transmitting envelope with electrical lead-in means sealed therethrough, the improvement which comprises:

(a) an incandescible filament principally comprising substantially stoichiometric tantalum carbide sup ported within said envelope and electrically connected to said lead-in means, said filament includes chemically combined nitrogen in amount of from 0.05 weight percent to 0.5 Weight percent taken with respect to the total weight of said filament; and

(b) a gaseous filling within said envelope and consisting essentially of nitrogen as the only reactive component, said nitrogen constituting from 0.4 volume percent to 100 volume percent of the total gaseous filling within said envelope, and any additional gaseous filling within said envelope other than nitrogen consisting of inert gas.

References Cited UNITED STATES PATENTS 2,928,977 3/1960 Roth et al 3 l3222 3,082,345 3/1963 Bottone 3l31l3 3,194,626 7/1965 Hofirnann 3l623 3,331,980 7/1967 Michael 3l3l13 3,411,959 11/1968 Corth 14813.l

RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R 

